Coaxial line circuit



I "MI-51R y 7, 1956 J. REINSMA 2,755,344

COAXIAL LINE CIRCUIT Filed Sept. 29, 1952 Fi 2. Q a6 l N V E NTO R Joy/v REM/5M4 ATTORNEY nited States Patent i 2,755,344 l a terrted July 17, 1956 COAXIAL LINE CIRCUIT John Reinsma, West Sayville, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Application September 29, 1952, Serial'No. 312,131

7 Claims. (Cl. 179-171) This invention relates to high frequency coaxial line circuits, and, more particularly, is concerned with means for coupling energy from a coaxial resonator to a coaxial output line.

Heretofore, energy has been coupled from a resonantcavity by means of probesor loops extending into the cavity region. However, where it is desired to tune the resonators over a considerable frequency band, for example, of the order of a two to one frequency band, probes and loops have not proved entirely satisfactory. Both probes and loops are frequency sensitive. Thus, the energy coupled out of the resonator by a probe or loop varies with frequency, and over an extended operating band, varies considerably unless the probe or loop is adjusted along with the tuning. of the resonator. For example, a probe or loop extending into a cavity resonator cannot be readily located at a point where the electric or magnetic field is maximum over the entire tunable band. As a result, the energy coupled from the resonator varies with changes in thetuned' frequency.

It is the general object of this invention to avoid the foregoing and other difficulties of and objections to the prior art practices by the provision of an improved coaxial line circuit having a resonant cavity type tuned circuit for operation over a broad frequency band by eliminating the need for coupling probes or loops.

Another object of this invention is to provide output coupling for a coaxial resonator which is always located at the voltage maximum of theresonator;

Another object of this invention is the provision of coupling means which is particularly adapted for use in coaxial resonator circuits such as grounded grid amplifiers and oscillators employing lighthouse tubes, i. e. disc-seal tubes with axial electron paths.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing in a coaxial line circuit including at lighthouse tube, an input coaxial line, the inner conductor and outer conductor of which are connected to the cathode and the control grid respectively of the tube. An output coaxial line is positioned in axial alignment: with the input line, the innerconductor of the output line being coupled to the anode of the tube and the outer conductor of the output line extending concentrically around the tube and a portion of the input line. A shortcircuiting annular plunger slidably positioned between the outer surface of the inputcoaxial line' and the inner surface of an extension of the outer conductor of the output coaxial line provides an adjustable end wall of a coaxial resonant cavity formed by the" concentric portion of the input and output coaxial lines. The opposite end of the cavity from the plunger provides output coupling from the cavity and comprises a pair of spaced annular plates separated by dielectric material to form an annular capacitor. One of the plates is supported from and electrically connected around its outer periphery to the inner surface of the outer conductor of the output line while the otherplat'e is connected to the anode of the tube.

For a better understanding of the invention, reference should be had to the accompanying drawing, where- Fig. l is a sectional view of asingle stage amplifier incorporating the capacitive output coupling feature of the present invention; and

Fig. 2 is a diagrammatic showing of the equivalent circuit for the coaxial circuit of Fig. 1.

Referring to Fig. 1 of the drawing, the numeral l9 indicates generally a coaxial amplifier circuit which includes an electron discharge device or vacuum tube 22 preferably adapted for very high or ultra high frequency operation, such as, for example, a lighthouse triode. The tube 12' is provided with an annular anode terminal 14, an annular grid terminal 16,. and a cathode terminal 18. The' cathode terminal is connected to ahollow conductor 20 by means of spring contact fingers 22'. The grid terminal in turn is connected to a hollow conductor 24 by means of an apertured' disc 25' having inner spring contact fingers thereon. Conductors 20 and 24 are held in concentric relationship by means of dielectric spacers 26 to form an input coaxial line section".

The output circuit of the tube 12 includes a coaxial resonant cavity 28- which is formed by providing an outer cylindrical conductive member 30 which is concentrically positioned by means of the annular? member- 31 around the conductor 24 and tube 12. One end wall of the resonant cavity 28 is provided by a shorting ring 32 in the form of a plungerwhich is sli'd'ably positioned between and in contact with the conductor 24* and the cylindrical member 30.

The other end Wall of the resonant cavity 28' is provided by an annular capacitorindicatedi generally at 3'4. One plate of the annular capacitor is provided by an annular disc 36 of conductive materialwhich is secured, for example, by being soldered around its outer periphery, to the inner surface of the cylindrical member 3% The other plate of the annular capacitor 34 is provided by a second annular disc 3% having an outer diameter slightly less than the inner diameter of the cylindrical member 30. The disc 38 is separated from the disc as by means of dielectric material such as mica, indicated at 40. The dielectric material is preferably bonded or otherwise secured to the two annular discsof the capacitor 34 to provide mechanical support of the annular disc 38 from the outer cylindrical member 30. Spring contact fingers 42 connect the anode' terminal 14 to the annular disc 38.

The output signal is coupled from the anode of the tube 12 by means of a cylindrical conductive member 44' which surrounds the coolingfins 46 of the tube 12'. Both the cylindrical members 30' and 44' are provided with slotted openings 47 in the region of the cooling fins of tube 12 to permit circulation of air for cooling. Cylindrical conductive member 4'4 is connected to the annular disc 38 of the capacitor 34, for example, by means of screws 49. The members 30 and 44 combine to provide an output coaxial line: section.

The other end of the cylindrical member 44. is connected to a disc 48 which forms one plate of an output coupling condenser 50, which: further includes a second plate 52 with dielectric material 54 between the plates 48 and 52. The inner conductor 55 of an output coaxial on the end of the cylindrical member 30. It should be noted that at frequencies where the cylindrical member 44 becomes longer than an A; wavelength, it is desirable to provide a tapered line section between the large diame ter coaxial line section formed by members 30 and 44 and the smaller diameter coaxial line connector 56 to obtain proper impedance matching to the load.

B+ voltage is applied to the anode of the tube 12 through an R. F. choke 60 and a conductor 62 which passes through a grommet 64 in the wall of the cylindrical member 30. An R. F. by-pass condenser 66 is provided to further isolate the B+ circuit from the R. F. of the coaxial line circuit 10. The condenser 50 isolates D.-C. voltage of the B+ circuit from the output signal.

A signal input to the coaxial line circuit 10 is provided through a coaxial connector 68 which is supported by means of an end plate 70, the plate 70 being secured to a flange 72 on the end of the outer cylindrical member 30 by means of screws 74. The inner conductor 76 of the input connector 68 is coupled through a condenser 78 to the conductor 20. The condenser 78 is formed by a disc 80 separated from a second disc 82 on the end of the conductor 20 by dielectric material 83.

Heater current to the tube 12 is provided from a transformer 84 through a pair of R. F. chokes 86 and through leads 88 which pass through an opening in the end plate 70. One of the leads is connected to the conductor 20, the other lead passing through a grommet 90 into the hollow center of the conductor 20 and being connected to the heater contact 92 of the tube 12. Grid bias is provided by a variable resistor 94 which is connected by means of the heater circuit between the cathode of the tube 12 and ground.

Tuning of the resonant cavity 28 is effected by means of rods 96 which are connected to the shorting ring 32 by passing through openings in the end plate 70 and the annular member 31. The rods 96 are joined by a yoke 98 to provide ready means for manually positioning the ring 32.

Referring to Fig. 2, the equivalent electrical circuit of the coaxial amplifier of Fig. 1 is shown diagrammatically. The cavity 28 of the coaxial line circuit of Fig. 1 is a capacity-loaded coaxial resonator, the shorted length of coaxial line being adjusted to less than a quarter wavelength or any odd multiple thereof so that its susceptance will resonate with the loading capacity at the desired operating frequency. Thus, the cavity is shown in Fig. 2 as an equivalent tuned inductance 28. The capacitive susceptance of the tuned circuit comprises the capacitor 34, which will be hereinafter referred to as having a capacity C1, in series with the grid-to-plate capacity Cgp of the tube 12. The coupling condenser 50 connects a load R1 across the capacitor 34, R being determined by the characteristic impedance of the output coaxial line.

Capacitor 34 thus in effect provides coupling between the resonant cavity 28 and the load R1, the voltage developed across capacitor 34 appearing across the load R1. The impedance Z1. into which the tube 12 operates is dependent on the ratio C1 to Cgp and the value of R1, and is given to a good approximation by the equation when R1 is much larger than X For example, with a Cg'p of 3 nnf. and an output coaxial line having a characteristic impedance of 50 ohms, the condenser 34 may have a capacity of 30 f. to give a load impedance of 5000 ohms. By changing the value of either R1 or C1, the Q of the resonator can be varied to control the bandwidth of the amplifier.

From the above description it will be recognized that the objects of the invention have been achieved by providing a power take-off arrangement for a coaxial line circuit which obviates the need for a coupling probe or loop in the coaxial resonator. By utilizing an annular capacitor, a symmetrical means of coupling is achieved, eliminating any distortion of the electromagnetic field Within the resonator cavity. Regardless of the tuning of the resonator, the coaxial output line to the load is always coupled to the resonator in the region of the voltage maximum, and broadband operation is greatly improved. Since there is no longer a need for pick-up loops or probes projecting into the resonator, the spacing between the inner and outer conductors can be greatly reduced for higher frequency operation, resulting in a smaller di ameter, more compact unit.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A coaxial line circuit comprising an ultra high frequency vacuum tube having a cathode terminal and annular grid and anode terminals spaced along an axis of the tube, a hollow cylindrical conductive member surrounding the tube and extending along said axis, annular capacitor means including a pair of annular plates separated by dielectric material, the periphery of one of the plates being in contact with the inner surface of the cylindrical member and the inner border of the other plate being in contact with said anode terminal, an output coaxial line section having an inner and an outer conductor, the inner conductor of the output coaxial line section being coupled to said anode terminal and extending along said axis of the tube, the cylindrical member being the outer conductor of the output coaxial line section, an input coaxial line section having an inner conductor and an outer conductor, the outer conductor of the input coaxial line section being connected to said grid terminal and extending along said axis of the tube in the opposite direction from the output coaxial line section and being supported concentrically within said cylindrical member, the inner conductor of the input coaxial line section being connected to said cathode terminal, and a shortcircuiting annular plunger slidably positioned between the outer surface of the input coaxial line section and the inner surface of the cylindrical member.

2. A coaxial line circuit comprising an ultra high fre quency tube having a cathode terminal and annular grid and anode terminals spaced along an axis of the tube, a hollow cylindrical conductive member surrounding the tube and extending along said axis, annular capacitor means including a pair of annular plates separated by dielectric material, one of the plates being joined around the periphery thereof to the inner surface of the cylindrical member and the other plate being joined around its inner border to said anode terminal, an output coaxial line section having an inner conductor and an outer conductor, the inner conductor of the output coaxial line section being coupled to said anode terminal and extending along said axis of the tube, the cylindrical member being the outer conductor of the output coaxial line section, a hollow cylindrical conductor concentric with the cylindrical member and extending along said axis of the tube in the opposite direction from the output coaxial line section, said cylindrical conductor being positioned within the cylindrical member and connected to said grid terminal, and means for shorting the cylindrical conductor to the cylindrical member at a point remote from the grid connected end of the cylindrical conductor.

3. A coaxial line circuit comprising an ultra high frequency tube having a cathode terminal and annular grid and anode terminals spaced along an axis of the tube. a hollow cylindrical conductive member surrounding the tube and extending along said axis, an annular capacitor extending radially between the cylindrical member and said anode terminal, an output coaxial line section having an inner conductor an an outer conductor, the inner conductor of the output coaxial line section being coupled to said anode terminal and extending along said axis of the tube, the cylindrical member being the outer conductor of the output coaxial linesection, and inductive means including a cylindrical conductor coaxial with and positioned within the cylindrical member, the cylindrical conductor being connected at one end to said grid terminal and shorted adjacent the other end of the cylindrical member.

4. In combination, an electron discharge device having a cathode terminal and annular grid and anode terminals, inductive means including a length of coaxial line having an inner conductor and outer conductor ter minated at one end in a short circuit, the electron discharge device being positioned adjacent the opposite end of said inner conductor from the shorted end and at least partially within the outer conductor, an annular capacitor extending between said annular anode terminal and said outer conductor, the inner conductor being connected to said grid terminal, an output coaxial line section, the outer conductor of said section being contiguous with the outer conductor of said inductive means and the inner conductor of said section being coupled to said annular anode terminal and means for coupling an alternating input signal between the grid and cathode terminals.

5. A tunable high frequency tube system comprising an electron discharge tube having a plurality of electrode terminals formed as mutually coaxial figures of revolution, an annular capacitor coaxial With said tube adjacent one electrode terminal thereof, said capacitor having first and second annular terminals, said first annular terminal being in contact with said one electrode terminal, a tunable coaxial line resonator having its outer conductor in contact with said second terminal of said capacitor and having its inner conducor coupled to a further electrode terminal of said tube, and means for transferring high frequency energy between said high frequency tube system and other high frequency devices, said last-named means comprising two-conductor energy conduit means, the conductors being coupled to the respective annular terminals of said annular capacitor for deriving an output from the high frequency voltage drop thereacross.

6. A tunable high frequency system for use in combination with an electron discharge device having a plurality of annular electrode terminals, said system comprising an annular capacitor having first and second annular terminals, a tunable coaxial line section having its outer conductor connected to said second terminal of said capacitor, the first annular terminal of said capacitor and the inner conductor of the coaxial line section being adapted to make contact with respective ones of said annular electrode terminals, and means for transferring high frequency energy between said high frequency system and other high frequency devices, said last-named means comprising two-conductor energy conduit means, the conductors being coupled to the respective annular terminals of said annular capacitor for deriving an output from the high frequency voltage drop thereacross.

7. A tuned amplifier comprising a vacuum tube having an anode and control grid, inductive means including coaxial inner and outer conductors terminated at one end in a short circuit, capacitive means including a pair of annular plates separated by dielectric material, one of the plates being connected to the end of said outer conductor remote from the short circuit, the end of said inner conductor remote from the short circuit being connected to the control grid of the vacuum tube, and the other plate of said capacitive means being connected to the anode of the vacuum tube, the inductive means combining with the capacitive means in series with the grid-to-anode capacity of the vacuum tube to form a resonant circuit, and means for coupling an output signal from said resonant circuit including a two-conductor transmission line, the conductors being connected respectively to the plates of said capacitive means.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,261 Whinnery July 16, 1946 2,416,322 Kellogg Feb. 25, 1947 2,421,784 Haeseler et al June 10, 1947 2,523,307 Kandoian Sept. 26, 1950 2,550,485 Lavoo et al. Apr. 24, 1951 2,551,715 Young May 8, 1951 2,642,533 Priest June 16, 1953 

